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The existence of SIS, established in 1909, was not formally acknowledged by the British government until 1994. The identity of the agency’s director, referred to only as “C,” after the first holder of the post, Capt. George Mansfield Smith-Cunningham, remained a secret for many years — albeit an increasingly poorly kept one — and Thursday’s was the first public address by a serving head of the agency.

It follows a similar debut earlier this month by the head of the British electronic and signals intelligence agency, Government Communications Headquarters. On Oct. 12, the agency’s director, Iain Lobban, spoke to a defense think tank in London.

Mr. Sawers, in a departure from the usual public tone struck by senior intelligence officials in Britain and elsewhere who normally praise the bravery of their own national intelligence operatives, highlighted the importance and the courage of the foreigners that were recruited by his agency.

“We at SIS obtain our intelligence from secret agents,” he said. But lest there be any confusion with James Bond or other fictional agents, he added, “these people are nearly all foreign nationals.”

These foreign agents “are the true heroes of our work,” he said, “working today in some of the most dangerous and exposed places, bravely and to hugely valuable effect.”

“Many of them show extraordinary courage and idealism,” he added, “striving in their own countries for the freedoms that we in Britain take for granted.”